53 research outputs found
Ocean changes in the North Atlantic over the Late Holocene: A multi-proxy approach
In today's North Atlantic, warm and salty surface waters are transported northwards from the subtropics, releasing their heat and eventually sinking to form a deep southward-flowing water mass. This process in the meridional-vertical plane is termed the Atlantic Meridional Overturning Circulation (AMOC). Changes in the strength and structure of the AMOC play a critical role in the Earthâs climate system and have been thought to be implicated in the climate variability of the Late Holocene.
In this thesis, two sediment cores RAPiD-35-25B and RAPiD-17-5P recovered from the Eirik Drift (South of Greenland) and the Iceland Basin are used to produce Late Holocene palaeoceanographic reconstructions of some of the main constituents of the AMOC at sub-decadal to multidecadal resolution.
Upper water column reconstructions from the Eastern Labrador Sea based on multi-species planktonic foraminiferal ÎŽ18O, Mg/Ca and faunal assemblages indicate millennial to centennial variability in the influence of polar waters reaching the Labrador Sea which likely led to reductions in deep water convection in the Labrador Sea. Inferred shifts of Labrador Sea Water formation appear concomitant with climatic anomalies recorded in the North Atlantic region. It is suggested that these climatic oscillations may have resulted from a coupled ocean-atmosphere response to reductions in solar irradiance.
The paired ÎŽ18O and Mg/Ca composition of the thermocline-dweller G. inflata, shows multi-centennial shifts in the temperature and salinity of the North Atlantic Current during the last millennium with a potential link to solar forcing. The recorded hydrographic variability is explained in terms of the effects of atmospheric blocking events on subpolar gyre dynamics.
Sortable silt mean grain size measurements show millennial-scale trends towards slower and faster vigour of the overflows east and west of Iceland during the Late Holocene. Potential upstream and downstream mechanisms are investigated and it is concluded that the millennial-scale variability in the strength of the overflow likely resulted from insolation driven Neoglacial changes in the freshwater budget and atmospheric circulation in the Nordic Seas. Furthermore, an antiphasing relationship between the strength of the two overflows is found, supporting previous predictions from modelling studies
Surface changes in the eastern Labrador Sea around the onset of the Little Ice Age
Despite the relative climate stability of the present interglacial, it has been punctuated by several centennial-scale climatic oscillations; the latest of which are often colloquially referred to as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA). The most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and/or volcanic activity and amplified by ocean-atmosphere-sea ice feedbacks. As such, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are widely believed to have been involved in the amplification of such climatic oscillations. The Labrador Sea is a key area of deep water formation. The waters produced here contribute approximately one third of the volume transport of the deep limb of the AMOC and drive changes in the North Atlantic surface hydrography and subpolar gyre circulation. In this study, we present multiproxy reconstructions from a high-resolution marine sediment core located south of Greenland that suggest an increase in the influence of polar waters reaching the Labrador Sea close to MCA-LIA transition. Changes in freshwater forcing may have reduced the formation of Labrador Sea Water and contributed toward the onset of the LIA cooling. © 2014. The Authors
Biological and climate controls on North Atlantic marine carbon dynamics over the last millennium: Insights from an absolutely-dated shell based record from the North Icelandic Shelf
Given the rapid increase in atmospheric carbon dioxide concentrations (pCO2) over the industrial era, there is a pressing need to construct longâterm records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the past biological and climate controls on the carbon isotopic (ÎŽ13Cshell) composition of the North Icelandic shelf waters over the last millennium, derived from the shells of the longâlived marine bivalve mollusk Arctica islandica. Variability in the annually resolved ÎŽ13Cshell record is dominated by multidecadal variability with a negative trend (â0.003 ± 0.002â° yrâ1) over the industrial era (1800â2000 Common Era). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (ÎŽ13C of CO2) derived from the burning of fossil fuels. Comparison of the ÎŽ13Cshell record with Contemporaneous proxy archives, over the last millennium, and instrumental data over the twentieth century, highlights that both biological (primary production) and physical environmental factors, such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and sea surface temperature and salinity of the subpolar gyre, are the likely mechanisms that contribute to natural variations in seawater ÎŽ13C variability on the North Icelandic shelf. Contrasting ÎŽ13C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated marine Suess effect signal at this locality
An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes
We investigate the mechanism of a decadal-scale weakening shift in the strength of the subpolar gyre (SPG) that is found in one among three last millennium simulations with a state-of-the-art Earth system model. The SPG shift triggers multicentennial anomalies in the North Atlantic climate driven by long-lasting internal feedbacks relating anomalous oceanic and atmospheric circulation, sea ice extent, and upper-ocean salinity in the Labrador Sea. Yet changes throughout or after the shift are not associated with a persistent weakening of the Atlantic Meridional Overturning Circulation or shifts in the North Atlantic Oscillation. The anomalous climate state of the North Atlantic simulated after the shift agrees well with climate reconstructions from within the area, which describe a transition between a stronger and weaker SPG during the relatively warm medieval climate and the cold Little Ice Age respectively. However, model and data differ in the timing of the onset. The simulated SPG shift is caused by a rapid increase in the freshwater export from the Arctic and associated freshening in the upper Labrador Sea. Such freshwater anomaly relates to prominent thickening of the Arctic sea ice, following the cluster of relatively small-magnitude volcanic eruptions by 1600 CE. Sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events; a necessary causal link between the volcanic cluster and the SPG shift can therefore be excluded. Instead, preconditioning by internal variability explains discrepancies in the timing between the simulated SPG shift and the reconstructed estimates for the Little Ice Age onset
A global database of Holocene paleotemperature records
A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.Fil: Kaufman, Darrell. Northern Arizona University.; Estados UnidosFil: McKay, Nicholas. Northern Arizona University.; Estados UnidosFil: Routson, Cody. Northern Arizona University.; Estados UnidosFil: Erb, Michael. Northern Arizona University.; Estados UnidosFil: Davis, Basil. University Of Lausanne; SuizaFil: Heiri, Oliver. University Of Basel; SuizaFil: Jaccard, Samuel. University Of Bern; SuizaFil: Tierney, Jessica. University of Arizona; Estados UnidosFil: DĂ€twyler, Christoph. University Of Bern; SuizaFil: Axford, Yarrow. Northwestern University; Estados UnidosFil: Brussel, Thomas. University of Utah; Estados UnidosFil: Cartapanis, Olivier. University Of Bern; SuizaFil: Chase, Brian. Universite de Montpellier; FranciaFil: Dawson, Andria. Mount Royal University; CanadĂĄFil: de Vernal, Anne. UniversitĂ© du QuĂ©bec a Montreal; CanadĂĄFil: Engels, Stefan. University of London; Reino UnidoFil: Jonkers, Lukas. University Of Bremen; AlemaniaFil: Marsicek, Jeremiah. University of Wisconsin-Madison; Estados UnidosFil: Moffa SĂĄnchez, Paola. University of Durham; Reino UnidoFil: Morrill, Carrie. University of Colorado; Estados UnidosFil: Orsi, Anais. UniversitĂ© Paris-Saclay; FranciaFil: Rehfeld, Kira. Heidelberg University; AlemaniaFil: Saunders, Krystyna. Australian Nuclear Science And Technology Organisation; AustraliaFil: Sommer, Philipp. University Of Lausanne; SuizaFil: Thomas, Elizabeth. University At Buffalo; Estados UnidosFil: Tonello, Marcela Sandra. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: TĂłth, MĂłnika. Balaton Limnological Institute; HungrĂaFil: Vachula, Richard. Brown University; Estados UnidosFil: Andreev, Andrei. Alfred Wegener Institut Helmholtz Centre for Polar and Marine Research; AlemaniaFil: Bertrand, Sebastien. Ghent University; BĂ©lgicaFil: Massaferro, Julieta. AdministraciĂłn de Parques Nacionales. Parque Nacional "Nahuel Huapi"; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
Winter amplification of the European Little Ice Age cooling by the subpolar gyre
Climate reconstructions reveal a strong winter amplification of the cooling over central and northern continental Europe during the Little Ice Age period (LIA, here defined as c. 16th-18th centuries) via persistent, blocked atmospheric conditions. Although various potential drivers have been suggested to explain the LIA cooling, no coherent mechanism has yet been proposed for this seasonal contrast. Here we demonstrate that such exceptional wintertime conditions arose from sea ice expansion and reduced ocean heat losses in the Nordic and Barents seas, driven by a multicentennial reduction in the northward heat transport by the subpolar gyre (SPG). However, these anomalous oceanic conditions were largely decoupled from the European atmospheric variability in summer. Our novel dynamical explanation is derived from analysis of an ensemble of last millennium climate simulations, and is supported by reconstructions of European temperatures and atmospheric circulation variability and North Atlantic/Arctic paleoceanographic conditions. We conclude that SPG-related internal climate feedbacks were responsible for the winter amplification of the European LIA cooling. Thus, characterization of SPG dynamics is essential for understanding multicentennial variations of the seasonal cycle in the European/North Atlantic sector
Multiple Equilibria as a Possible Mechanism for Decadal Variability in the North Atlantic Ocean
Data Descriptor: A global multiproxy database for temperature reconstructions of the Common Era
Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850-2014. Global temperature composites show a remarkable degree of coherence between high-and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.(TABLE)Since the pioneering work of D'Arrigo and Jacoby1-3, as well as Mann et al. 4,5, temperature reconstructions of the Common Era have become a key component of climate assessments6-9. Such reconstructions depend strongly on the composition of the underlying network of climate proxies10, and it is therefore critical for the climate community to have access to a community-vetted, quality-controlled database of temperature-sensitive records stored in a self-describing format. The Past Global Changes (PAGES) 2k consortium, a self-organized, international group of experts, recently assembled such a database, and used it to reconstruct surface temperature over continental-scale regions11 (hereafter, ` PAGES2k-2013').This data descriptor presents version 2.0.0 of the PAGES2k proxy temperature database (Data Citation 1). It augments the PAGES2k-2013 collection of terrestrial records with marine records assembled by the Ocean2k working group at centennial12 and annual13 time scales. In addition to these previously published data compilations, this version includes substantially more records, extensive new metadata, and validation. Furthermore, the selection criteria for records included in this version are applied more uniformly and transparently across regions, resulting in a more cohesive data product.This data descriptor describes the contents of the database, the criteria for inclusion, and quantifies the relation of each record with instrumental temperature. In addition, the paleotemperature time series are summarized as composites to highlight the most salient decadal-to centennial-scale behaviour of the dataset and check mutual consistency between paleoclimate archives. We provide extensive Matlab code to probe the database-processing, filtering and aggregating it in various ways to investigate temperature variability over the Common Era. The unique approach to data stewardship and code-sharing employed here is designed to enable an unprecedented scale of investigation of the temperature history of the Common Era, by the scientific community and citizen-scientists alike
Search for High-energy Neutrinos from Binary Neutron Star Merger GW170817 with ANTARES, IceCube, and the Pierre Auger Observatory
Multi-messenger Observations of a Binary Neutron Star Merger
On 2017 August 17 a binary neutron star coalescence candidate (later
designated GW170817) with merger time 12:41:04 UTC was observed through
gravitational waves by the Advanced LIGO and Advanced Virgo detectors.
The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray
burst (GRB 170817A) with a time delay of ⌠1.7 {{s}} with respect to
the merger time. From the gravitational-wave signal, the source was
initially localized to a sky region of 31 deg2 at a
luminosity distance of {40}-8+8 Mpc and with
component masses consistent with neutron stars. The component masses
were later measured to be in the range 0.86 to 2.26 {M}ÈŻ
. An extensive observing campaign was launched across the
electromagnetic spectrum leading to the discovery of a bright optical
transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC
4993 (at ⌠40 {{Mpc}}) less than 11 hours after the merger by the
One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The
optical transient was independently detected by multiple teams within an
hour. Subsequent observations targeted the object and its environment.
Early ultraviolet observations revealed a blue transient that faded
within 48 hours. Optical and infrared observations showed a redward
evolution over âŒ10 days. Following early non-detections, X-ray and
radio emission were discovered at the transientâs position ⌠9
and ⌠16 days, respectively, after the merger. Both the X-ray and
radio emission likely arise from a physical process that is distinct
from the one that generates the UV/optical/near-infrared emission. No
ultra-high-energy gamma-rays and no neutrino candidates consistent with
the source were found in follow-up searches. These observations support
the hypothesis that GW170817 was produced by the merger of two neutron
stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and
a kilonova/macronova powered by the radioactive decay of r-process
nuclei synthesized in the ejecta.</p
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